Modern communication and remote sensing systems have become very sophisticated and effective. In the communications fields, equipment provides highly reliable, and in some cases secure, communications between or among widely separated locations, some or all of which may be in motion. One of the technological advancements which has allowed the excellent communications is the ubiquitous solid-state device, which has supplanted older technologies such as vacuum tubes. Among the major advantages of solid-state devices are small size, which allows complex functions to be incorporated into portable packages, and lack of any inherent degradation mechanisms. A salient disadvantage of such devices arises from the small size, and that disadvantage is the sensitivity of solid-state devices to application of small amounts of energy or power. It is well known that solid-state devices can be destroyed during those times in which they are being installed in a circuit by static discharges. Such static discharges can arise even from physical movements of the installer or his clothing, or from walking across a floor. Wrist straps are often provided to help to reduce static buildup when devices are being worked on.
Even when solid-state devices are properly installed in their circuits, they are still more vulnerable to certain types of damage than are vacuum tubes. More particularly, the input stages of certain equipments, such as low-noise amplifiers, are subject to damage similar to that occasioned by static discharge in the presence of strong electrical signals. Such strong electrical signals can occur even in sophisticated equipment. For example, unwanted signals can enter an antenna which happens to be in the presence of the unwanted signals. Even in “closed” systems such as a cable television system, unwanted signals, which may be of significant amplitude, may enter through imperfections in the joints between cable sections, or, in the case of a lightning strike, through the walls of the enclosure. In order to reduce the likelihood of damage to solid-state equipment subject to unwanted signals, the prior art uses bandwidth-reducing filters, such as bandpass filters, to tend to reject signals which are not in the frequency range(s) of interest. Alternatively or in addition, voltage limiters can be connected to the input port of sensitive equipment, to thereby tend to reduce the amplitude of unwanted signals of large amplitude.
The proliferation of communications and sensing devices which are difficult or impossible to service, such as microwave repeaters which are located on remote mountaintops, or communications links or sensors using Earth satellites, has increased the importance of protection of solid-state devices against unwanted electrical or electromagnetic surges. This is especially so when attacks against communication systems may include electromagnetic pulse generators such as nuclear blasts.
Improved or alternative protection is desired of electrically or electromagnetically sensitive equipment.